A copolymer of an alkyl vinyl ether/maleic anhydride, in particular, a copolymer of methyl vinyl ether/maleic anhydride is a straight chain and water-soluble polymeric electrolyte compound, and recently, has been widely put into use in such industrial fields as additives to a sticking agent (a sticking plaster), an adhesive, a preventive agent for adhesion of detergents, a spray-type fixing agent for hair, a builder for synthetic detergents, a dental material or the like, because the copolymer is excellent in nonpoisonous properties to the human body, stability for a long period of time, tackiness, cohesiveness, a water-holding property, a peeling property and the like. Further, an esterification-product of the maleic anhydride portion in the copolymer, obtained by reacting the copolymer with various alcohols, is also used in such fields.
Heretofore, it has been well known that an alkyl vinyl ether is copolymerized with maleic anhydride in various solvents, in the presence of a radical initiator, to afford a copolymer of alkyl vinyl ether/maleic anhydride. In this case, such a reaction method as to effect a precipitation-polymerization in a solvent which can dissolve a raw material maleic anhydride but can not dissolve the copolymer of alkyl vinyl ether/maleic anhydride, and as to effect a solution-polymerization in a solvent which can dissolve both of the raw material maleic anhydride and the copolymer of an alkyl vinyl ether/maleic anhydride have been well known.
Examples of the precipitation-polymerization method include a method to effect a reaction using, as a solvent, an aromatic hydrocarbon such as benzene (e.g., U.S. Pat. Nos. 2,782,182, 3,030,343, 3,499,876, 3,532,771, 3,553,183, DE3302495, JP-A-01204910, JP-A-01204911, and JP-A-01204912, U.S. Pat. No. 4,900,809 etc.), toluene or xylene (U.S. Pat. Nos. 3,499,876, 3,532,771, 3,553,183, DE3302495, JP-A-01204910, JP-A-01204911 and JP-A-01204912 etc.); an aliphatic hydrocarbon such as hexane, heptane or octane (U.S. Pat. No. 3,532,771); a halogenated aliphatic hydrocarbon such as dichloroethane (GB 1117515, U.S. Pat. No. 3,499,876, DE3302495 etc.), trichloroethane, carbon tetrachloride (DE3302495) or a mixture of 1,2-trichloro-1,2,2-trifluoroethane/dichloromethane (JP-A-55145714); ester such as an ester having not less than 5 of carbon atoms (DE3736996), isopropyl acetate or methyl propionate (JP-A-01204910, JP-A-01204911 and JP-A-01204912); and ether such as allyl vinyl ether (GB 906230, DE-3712265, DE-3108100) or a monoether which is sterically bulky (WO-9012817).
Examples of the solution -polymerization method include a method to effect a reaction using, as a solvent, methyl acetate, ethyl acetate (U.S. patent application Ser. No. 334404), acetone (U.S. Pat. Nos. 3,499,876, 4,948,848), methyl ethyl ketone (U.S. Pat. No. 3,499,876) and the like.
However, it has been difficult to get near zero with respect to a solvent concentration remaining in a final product, even though drying of the copolymer solution of an alkyl vinyl ether/maleic anhydride is carefully conducted in a drying process after completion of polymerization thereof, in case when a polymerization reaction is carried out, at a usual temperature, by the use of a harmless solvent to the human body such as an organic acid ester like ethyl acetate or such as a ketone base solvent like methyl ethyl ketone, in a usual technology, in place of a harmful solvent such as benzene, toluene, xylene, carbon tetrachloride, dichloroethane or trichloroethane.
In case when a detergent, a sticking agent or a fixing agent for hair, to which a copolymer of an alkyl vinyl ether/maleic anhydride has been added, is employed, these agents directly come in contact with the human body (the skin etc.). Accordingly, it has been demanded to develop a reaction method to use a solvent which is harmless to the human body, even if the above-mentioned agents come in contact with the human body, and demanded also to be able to decrease concentration of a solvent remaining in a final product. Reaction methods using solvents other than the aforesaid solvents are also known. It is, however, noted that these methods bring about such drawbacks as described in the following.
For example, when such an ester as ethyl formate, isopropyl acetate, butyl acetate or methyl propionate is used, a copolymer produced during a reaction will become liable to adhere to the wall of a reaction vessel, which causes a problem in handling. On the other hand, when a reaction is conducted by a solution-polymerization method using methyl acetate, ethyl acetate, acetone, methyl ethyl ketone or the like as disclosed in the foregoing prior arts, a lot of energy will be required in removal of a solvent as compared with the case of a precipitation-polymerization method. As to a reaction method, using such an expensive solvent as a mixture of 1,1,2-trichloro-1,2,2-trifluoroethane/dichloromethane, monoether which is sterically bulky, or allyl vinyl ether, it can be said that the method will pose a cost problem in view of some losses inevitably derived in case of a solvent removal. Further, when only an aliphatic hydrocarbon such as hexane, heptane or octane is used, a monomer will become difficult to be dissolved in a solvent and a reaction will become difficult to proceed, accordingly.
Furthermore, in case of a method for producing a powdery copolymer of methyl vinyl ether/maleic anhydride using ethyl acetate or acetone as disclosed in JP-A-04504869 Official Gazette, it is necessary to use too much excessive methylvinyl ether with respect to the maleic anhydride in order to maintain a slurry state, and therefore, it is required to spend a lot of cost in a compressed reaction system and in recovery or re-use of the too much excessive methyl vinyl ether, which can be said that the method is problematic.
The present inventors proposed a method for producing a copolymer of methyl vinyl ether/maleic anhydride having a high specific viscosity as disclosed in JP-A-12191705 Official Gazette. In the method, the copolymer having a high specific viscosity is produced, by employing a divided and dropwise adding way of alkyl vinyl ether into the raw material maleic anhydride preliminarily dissolved in a solvent such as benzene. Namely, in the method, the contact time between the maleic anhydride and the copolymer of methyl vinyl ether/maleic anhydride is taken into consideration, and basically speaking, the method is a precipitation-polymerization to use a benzene series solvent.
However, an idea to precipitate a copolymer, using a solvent which dissolves the copolymer of an alkyl vinyl ether/maleic anhydride, and making use of a phenomenon that the solubility of copolymer lowers by means of heating, after completion of the polymerization, has never been disclosed in any of the prior arts, and it can be said that the method according to the present invention is an epoch-making one.
As to the copolymer of an alkyl vinyl ether/maleic anhydride which has been used in various objects as mentioned hereinbefore, a suitable molecular weight range thereof is selected according to the objects. However, the molecular weight thereof is low on the whole, and it has been desired to get a high molecular weight copolymer in order to improve performances for each different use.
As to the method for producing a copolymer of an alkyl vinyl ether/maleic anhydride, a variety of means has been disclosed so far, and for example, JP-A-03221511, JP-A-3215509, JP-A-5509341, and JP-A-12191705 Official Gazettes are cited in addition to the hereinbefore mentioned related arts. However, all of these means is concerned with proposed conditions for a reaction process, and does not disclose any of a removing method for solvents used in the reaction, and a method for drying and the like.